System and method to identify regions of airspace having ice crystals using an onboard weather radar system

ABSTRACT

Systems and methods of detecting type I ice crystals using an aircraft&#39;s onboard weather radar system are disclosed. An exemplary embodiment identifies radar returns having a return level signal strength less than a radar return sensitivity threshold level, determines if at least one of a weather condition and a flight condition concurrently exists with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level, and identifies a region of airspace potentially having type I ice crystals when the at least one of the weather condition and the flight condition concurrently exists with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level.

PRIORITY CLAIM

This application claims priority to Provisional Patent Application Ser.No. 61/608,562 filed on Mar. 8, 2012 and is incorporated herein byreference.

BACKGROUND OF THE INVENTION

High altitude ice crystals that are not visible to the crew of anaircraft, especially at night, may present a hazard to the aircraftpassing through an airspace region having high concentrations of the icecrystals. Such ice crystals, which may be encountered while flyingthrough clouds, may cause power loss in a jet engine or even enginedamage. Power loss may include engine instability, power surge, powerrollback, stalling, and/or flameout.

These non-visible ice crystals may be found at altitudes between 9,000and 40,000 feet, when the ambient temperature is between −5° C. to −55°C. High altitude ice crystals are typically associated with convectivecloud formations, such as isolated cumulonimbus cloud formations,thunderstorms, or even tropical squall lines. Such convective cloudformations can contain areas of strong updrafts. The strong updrafts inthe convective weather can lift liquid water thousands of feet into theatmosphere, and thus form ice crystals.

Regions of airspace having non-visible ice crystals may be at altitudesthat are significantly higher than the visible tops of the convectivecloud formations which formed the ice crystals. Further, high altitudewinds may blow the ice crystals away from the convective cloudformations. Accordingly, regions of airspace having large concentrationsof ice crystals may be encountered far above and/or away from thevicinity of the convective cloud formations which formed the icecrystals. Further, such airspace regions may have little to noturbulence to provide a warning indication to the crew of an aircraft asit flies into an airspace region with high concentrations of icecrystals.

Because of the very small size of the ice crystals, which may be assmall as 40 microns, the ice crystals are not visibly detectable by theaircraft crew. Thus, the crew may not be aware that their aircraft istravelling through an airspace region having a high concentration of icecrystals, particularly if there is no turbulence and/or if the airspaceregion is not near any convective cloud formations.

Further, conventional aircraft weather radar systems are typically notconfigured to detect ice crystals since the reflectivity levels ofreturn echoes from the ice crystals and/or the associated cloud cover,which may be in the range of 0-20 decibels (dBZ), is below the radarreturn sensitivity threshold levels, typically set at or about 20 dBZ.The aircraft weather radar systems are configured to detect larger waterparticles, such as rain, hail, and/or snow since these larger waterparticles provide radar reflectivity signal strength return levels thatare discernable by the aircraft weather radar systems since such radarreflectivity signal strength return levels exceed the minimum radarreturn sensitivity threshold levels of approximately 20 dBZ. Low levelradar reflectivity signal strength return levels that are less than aradar return sensitivity threshold level are typically ignored to reducethe amount of information presented on the radar system display. Thatis, since low levels of precipitation are typically not dangerous toaircraft, and since regions with low levels of precipitation exhibit lowradar reflectivity signal strength return levels below the 20 dBZthreshold, such regions of low levels of precipitation are typically notindicated on the radar system display to avoid distracting the aircraftcrew. Accordingly, the presence of ice crystals are not indicated on theradar system display because of their low radar reflectivity signalstrength return levels.

Accordingly, there is a need in the arts to provide the crew of anaircraft information indicating the location of regions of airspace thatare likely to have ice crystals so that the crew may then avoid airspacehaving potentially high concentrations of ice crystals.

SUMMARY OF THE INVENTION

An exemplary embodiment of a type I ice crystal display systemidentifies type I ice crystals using an aircraft's onboard weather radarsystem. An exemplary embodiment identifies radar returns having a returnlevel signal strength less than a radar return sensitivity thresholdlevel, determines if at least one of a weather condition and a flightcondition concurrently exists with the identified radar returns havingthe return level signal strength less than the radar return sensitivitythreshold level, and identifies a region of airspace potentially havingtype I ice crystals when the at least one of the weather condition andthe flight condition concurrently exists with the identified radarreturns having the return level signal strength less than the radarreturn sensitivity threshold level.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred and alternative embodiments are described in detail below withreference to the following drawings:

FIG. 1 is a perspective view of a portion of a planned flight path of anaircraft traveling through a region of airspace having a convectivestorm cell and a region of airspace with type I crystals;

FIG. 2 is a block diagram of an embodiment of the type I ice crystaldisplay system implemented in an aviation electronics system of theaircraft;

FIG. 3 is a display image presenting a plan view of the planned flightpath through the region of airspace having convective weather prior todetection of the region of airspace 108 potentially having type I icecrystals; and

FIG. 4 is a display image presenting a plan view of the planned flightpath through the region of airspace having convective weather after todetection of the region of airspace potentially having type I icecrystals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a perspective view of a portion of a planned flight path of anaircraft 102 traveling through a region of airspace 104 havingconvective cloud weather 106 and traveling through a region of airspace108 with type I crystals 110. The aircraft includes an onboard weatherradar system with an embodiment of a type I ice crystal display system100. For clarity, ice crystals are defined as either type I ice crystalsor type II ice crystals herein.

Type I ice crystals are defined herein as ice crystals typically foundat altitudes between 9,000 and 40,000 feet, and/or when the ambienttemperature is between −5° C. to −55° C. These high altitude type I icecrystals are associated with the convective cloud weather 106. Type Iice crystals are often not visible to the crew of the aircraft 102, andmay be encountered while flying through relatively light cloud cover.Type I ice crystals are not associated with liquid water droplets. Thatis, liquid water droplets are not present with type I ice crystals.Accordingly, type I ice crystals do not cause significant icing onaircraft surfaces, such as the aircraft wings. Rather, type I icecrystals may cause potential engine failure or power loss. Reflectivitylevels of weather radar return echoes from type I ice crystals aregenerally in the range of 10-20 decibels (dBZ), which is below the radarreturn sensitivity threshold levels of an onboard weather radar system,typically set at or about 20 dBZ.

Type II ice crystals, in contrast, are often found with and areassociated with liquid water droplets in the convective cloud weather106. Under certain conditions, type II ice crystals and the associatedliquid water droplets may cause hazardous icing on aircraft surfaces,such as the aircraft wings. The region of airspace associated with typeII ice crystals and the associated liquid water droplets often willexhibit reflectivity levels of weather radar return echoes that aregreater than that of the type I ice crystals, and may often exceed the20 dBZ radar return sensitivity threshold level of the onboard weatherradar system. Even if the reflectivity signal strength returns fromregions of airspace having high concentrations of the type II icecrystals and liquid water is less than the display threshold ofreflectivity signal strength returns of the onboard weather radarsystem, the detection and/or notification of the crew of the aircraft102 is not the primary interest of embodiments of the type I ice crystaldisplay system 100. That is, embodiments of the type I ice crystaldisplay system 100 are primarily concerned with the detection and theattendant crew notification of regions of airspace 108 having type I icecrystals 110 which may cause engine failure or loss of power, and notnecessarily detection and notification for type II ice crystals that maycause icing conditions of the surface of the aircraft 102.

Embodiments of the type I ice crystal display system 100 may beconfigured to receive input from an altitude detector, which may be asensor or other altitude determination system (not shown). The altitudedetector provides information corresponding to the aircraft's altitude.The altitude of the aircraft 102 may correspond to a region of airspace108 that is likely to have type I ice crystals 110. The altitude may bereferenced to sea level or a height above ground. Any suitable altitudedetector may be used. The information provided by the altitude detectormay be directly provided to the onboard weather radar system and/or thetype I ice crystal display system 100, and/or may be provided via otherintermediary systems which are receiving and/or determining the altitudeinformation

Example embodiments of the type I ice crystal display system 100 may beconfigured to receive input from an ambient air temperature detector,which may be a sensor or other temperature determination system (notshown). Alternatively, or additionally, embodiments may be configured toestimate temperature at the altitude and/or location of the region ofairspace 108 that may have the type I crystals 110. The ambient airtemperature detector provides information corresponding to the ambientair temperature outside of the aircraft. That is, the temperature thatthe aircraft 102 is currently flying through, and/or an estimatedtemperature of a region of airspace that is likely to have type I icecrystals, may be determined from information provided by the airtemperature detector. The temperature may be referenced to degreeCelsius or degrees Farenheight. Any suitable ambient air temperaturedetector may be used. The information provided by the ambient airtemperature detector may be directly provided to the onboard weatherradar system and/or the type I ice crystal display system 100, and/ormay be provided via other intermediary systems which are receivingand/or are determining the temperature information.

Additionally, embodiments of the type I ice crystal display system 100may be configured to receive radar reflectivity signal strength returninformation from the onboard weather radar system. The type I icecrystal display system 100 monitors for low level reflectivity signalstrength returns received by the onboard weather radar system. Such lowlevel reflectivity signal strength returns may be indicative of theregion of airspace 108 having type I ice crystals 110, particularly ifother weather conditions and/or flight conditions concurrently exist.

Type I ice crystals and any associated cloud cover are known to providelow level reflectivity signal strength returns, which are typicallybelow a radar reflectivity display threshold which defines a minimumlevel of reflectivity signal strength returns that are displayed by theonboard weather radar system. For example, the onboard weather radarsystem may have a radar reflectivity display threshold set at 20 dBZ.(However, the radar reflectivity display threshold of reflectivitysignal strength returns may be set at any level of interest.) When theonboard weather radar system detects reflectivity signal strengthreturns above the radar reflectivity display threshold, the onboardweather radar system determines that weather or ground objects have beendetected. Accordingly, a display of the onboard weather radar systempresents the information to the crew of the aircraft 102. Severity ofthe detected weather may be indicated on the display using differentcolors, such as yellow for weaker strength weather, or red for moresevere strength weather.

When low level reflectivity signal strength returns are detected by theonboard weather radar system concurrently with one or more other weatherconditions and/or flight conditions, the type I ice crystal displaysystem 100 may identify a region of airspace 108 potentially having typeI ice crystals 110. In response to determining that there is alikelihood of the presence of a region of airspace having a highconcentration of type I ice crystals, the embodiments of the type I icecrystal display system 100 notify or otherwise advise the crew of theaircraft 102. Accordingly, the crew may change the flight path 112 ofthe aircraft 102 to avoid the identified region of airspace 108potentially having type I ice crystals 110.

Accordingly, embodiments of the type I ice crystal display system 100assess the likelihood that a region of airspace may potentially havetype I ice crystals when the radar returns from that region of airspaceare less than a predefined reflectivity display threshold value or arange of the radar reflectivity display threshold values. For example,an embodiment may monitor for reflectivity signal strength returns ofless than the radar reflectivity display threshold, such as 20 dBZ.

Alternatively, an embodiment may monitor for reflectivity signalstrength returns of less than another predefined value that may be lessthan, or even greater than, the radar reflectivity display threshold.For example, but not limited to, an embodiment may monitor forreflectivity signal strength returns that are less than 10 dBZ.

Alternatively, an embodiment may monitor for reflectivity signalstrength returns of less than the radar reflectivity display threshold(or another predefined value) and that are greater than a predefinedminimum value. For example, but not limited to, an embodiment maymonitor for reflectivity signal strength returns in a range between 5dBA and 10 dBZ.

As noted above, when one or more weather conditions and/or flightconditions concurrently occur along with the detection low levelreflectivity signal strength returns by the onboard weather radarsystem, embodiments of the type I ice crystal display system 100 maydetermine that type I ice crystals may be present in the associatedregion of airspace. These weather conditions and/or flight conditionsinclude:

-   -   a location of a region of airspace 108 from which the low level        reflectivity signal strength returns originated being less than        or equal to a maximum range threshold 114 from the aircraft 102;    -   a location of a region of airspace 108 from which the low level        reflectivity signal strength returns originated being less than        or equal to a minimum range threshold 116 from the aircraft 102;    -   an ambient air temperature being less than or equal to an        ambient air temperature threshold;    -   an altitude of the aircraft 102 being greater than or equal to        an aircraft altitude threshold 118;    -   an altitude of a region of airspace 108 from which the low level        reflectivity signal strength returns originated being greater        than or equal to a type I ice crystal altitude threshold 120;    -   a distance between a region of airspace 108 from which the low        level reflectivity signal strength returns originated and        detected convective weather 122 being less than or equal to a        convective weather proximity threshold 124;    -   a size of detected convective weather 122 being greater than or        equal to a convective weather size threshold;    -   an intensity of detected convective weather 122 being greater        than or equal to a convective weather intensity threshold; and    -   a vertically integrated liquid-water (VIL) content value being        greater than or equal to a VIL content value threshold.

As noted above, a weather condition that may be considered concurrentlywith the above-described detection low level reflectivity signalstrength returns by the onboard weather radar system is the range of theregion of airspace (distance from the aircraft 102) from which the lowlevel reflectivity signal strength returns originated. If the locationof the region of airspace from which the low level reflectivity signalstrength returns originated is less than or equal to a maximum rangethreshold 114 from the aircraft 102, embodiments of the type I icecrystal display system 100 may identify a region of airspace 108potentially having type I ice crystals 110. The identified region ofairspace 108 with low level reflectivity signal strength returns andpotentially having type I ice crystals is then indicated to the crew ofthe aircraft 102.

It is appreciated that when weather or other structure reflects incidentradar signals, the reflected radar signal portion that is returningtowards the aircraft disperses. This phenomenon is commonly referred toas “space loss” in the arts. Accordingly, relatively intense weather mayinitially reflect incident radar signals at a strength that exceeds 20dBZ. However, if such weather is relatively far away from the aircraft102, the reflectivity signal strength returns originating from therelatively far away weather that are eventually detected by the onboardweather radar system may have decreased below 20 dBZ radar reflectivitydisplay threshold. If such weather had been at a closer range to theaircraft 102, then the detected reflectivity signal strength returnswould have been in excess of the radar reflectivity display threshold,and accordingly, would have been indicated on the display of the onboardweather radar system.

In contrast, if the low level reflectivity signal strength returnsoriginate from a region of airspace that is at a relatively close to theaircraft 102, it is likely that such returns may be due to eitherreflectivity signal strength returns from very light precipitation,light cloud cover, and/or type I ice crystals. Accordingly, regions ofairspace associated with the low level reflectivity signal strengthreturns must be within a maximum range threshold 114 of the aircraft 102to be considered as a region of airspace 108 of interest potentiallyhaving type I ice crystals 110. In an exemplary embodiment, the maximumrange threshold 114 is 20 miles. Other embodiments may use any suitablemaximum range threshold 114 value. For example, but not limited to, ifthe region of airspace associated with the low level reflectivity signalstrength returns is at a range of 15 miles out from the aircraft 102,then the type I ice crystal display system 100 may determine that thatparticular region of airspace may have type I ice crystals.

Further, some embodiments may automatically adjust the maximum rangethreshold 114 based on various weather conditions and/or flightconditions. Alternatively, or additionally, the maximum range threshold114 may be manually adjusted by the crew of the aircraft 102. Forexample, but not limited to, if the sky is generally clear of convectiveweather, the maximum range threshold 114 may be adjusted to a firstrange. On the other hand, if convective weather is close to theaircraft, the maximum range threshold 114 may be adjusted to a secondrange. In an example embodiment, if the sky is clear and convectiveweather is not proximate to the aircraft 102, the first maximum rangethreshold 114 may be set to 40 miles. When convective weather isproximate to the aircraft 102, then the maximum range threshold 114 maybe adjusted to 20 miles.

Additionally, or alternatively, a minimum range threshold 116 may beemployed by embodiments of the type I ice crystal display system 100. Iflow level reflectivity signal strength returns originate from a regionof airspace that is less than or equal to the minimum range threshold116, such low level reflectivity signal strength returns aredisregarded, and accordingly, are not indicated to the crew of theaircraft 102. For example, but not limited to, a minimum range threshold116 of 5 miles may be employed. In this simplified example, if a regionof airspace originating the low level reflectivity signal strengthreturns is detected at a 4 mile range, it may not be possible for thecrew to adjust the flight path 112 in time to avoid that region ofairspace. Accordingly, that region of airspace 108 potentially havingtype I ice crystals 110 may not be indicated to the crew. (However,regions of airspace 108 potentially having type I ice crystals 110 at arange of 5 miles or more are of interest and would be indicated to thecrew.)

Alternatively, when a region of airspace 108 originating the low levelreflectivity signal strength returns is detected that is at a range thatis less than or equal to the minimum range threshold 116, the region ofairspace 108 may be indicated to the crew with a heightened level ofawareness. For example, but not limited to, an audible alarm or warningmay be issued. Alternatively, or additionally, the region of airspace108 may be indicated on the weather radar display with a colorassociated with a heightened awareness color, with flashing, or thelike. For example, but not limited to, the region of airspace 108potentially having type I ice crystals may be indicated using a magentacolor or other color that the standardized red/yellow/green colors usedby legacy weather radar display systems. Alternatively, or additionally,a special pattern or fill (such as, but not limited to, a paisleypattern) may be used to indicate the region of airspace 108 potentiallyhaving type I ice crystals. In other embodiments, the reflectivitythreshold applicable to regions indicated using a green display colormay be lowered below the normal operating/display threshold.Accordingly, the crew of the aircraft 102 may appreciate that apotential engine failure or power loss may be imminent in view of apossible incursion into the region of airspace 108 potentially havingtype I ice crystals, and therefore, may provide the crew with a periodof time to prepare for or take remedial actions.

Another weather condition that may be considered concurrently with thelow level reflectivity signal strength returns detected by the onboardweather radar system is the ambient temperature. If the ambienttemperature is greater than or equal to an ambient air temperaturethreshold, type I ice crystals will not likely be present. If theambient air temperature is less than or equal to the ambient airtemperature threshold, then type I ice crystals may be present.Accordingly, if a region of airspace 108 with low level reflectivitysignal strength returns is detected and the ambient temperature is lessthan or equal to the ambient air temperature threshold, then embodimentsof the type I ice crystal display system 100 may determine that type Iice crystals may be present in the associated region of airspace 108. Inan example embodiment, the ambient air temperature threshold is −40° C.Any suitable ambient air temperature threshold may be used.

In addition to sensing ambient temperature outside of the aircraft 104,some embodiments may optionally, or additionally, estimate an ambienttemperature for an identified region of airspace 108 with low levelreflectivity signal strength returns. For example, an ambienttemperature-altitude gradient estimation may be used to estimate ambienttemperature, particularly when the altitude of the aircraft 102 isdifferent from the altitude of the region of airspace 108 with low levelreflectivity signal strength returns.

Alternatively, or additionally, information pertaining to temperature atthe altitude and/or location of the region of airspace 108 potentiallyhaving type I ice crystals may be obtained from other sources. Forexample, but not limited to, temperature information may be receivedfrom other aircraft. Alternatively, or additionally, temperatureinformation may be received from a groundstation or other terrestrialsource.

In an example embodiment, the ambient temperature-altitude gradient maybe a standardized or predefined curvilinear function (such as a straightor a curved line). In some embodiments, the intercept of the ambienttemperature-altitude gradient may be adjustable based on the currentambient temperature about the aircraft 102. In some embodiments, theambient temperature-altitude gradient may be learned. For example, theambient temperature-altitude gradient may be learned if the aircraft 102has changed altitudes and detected ambient temperatures about theaircraft 102 at various altitudes has been saved. Thus, a slope and/oran intercept of the ambient temperature-altitude gradient may bedetermined based on actual ambient temperature data.

Another flight condition that may be considered concurrently with thelow level reflectivity signal strength returns detected by the onboardweather radar system is an aircraft altitude threshold 118. If thecurrent altitude of the aircraft 102 is greater than or equal to theaircraft altitude threshold 118, then embodiments of the type I icecrystal display system 100 may determine that type I ice crystals may bepresent in the associated region of airspace 108. An noted above, analtitude detector may provide a current altitude of the aircraft 102. Inan example embodiment, the aircraft altitude threshold may be 9,000 feetabove sea level, though any aircraft altitude threshold 118 may be used.Further, the altitude threshold 118 may be adjustable to account forother weather conditions and/or flight conditions. For example, but notlimited to, a first aircraft altitude threshold 118 may be used when theaircraft 102 is flying over an ocean, and a second aircraft altitudethreshold 118 may be used when the aircraft 102 is flying overmountains.

Another weather condition that may be considered concurrently with thelow level reflectivity signal strength returns detected by the onboardweather radar system is a type I ice crystal altitude threshold 120.Altitude of the region of airspace 108 associated with the low levelreflectivity signal strength returns may be computed based upon a rangeand elevation as determined by the onboard weather radar system withrespect to the current altitude of the aircraft 102. In an exampleembodiment, a type I ice crystal altitude threshold may be 9,000 feetabove sea level, though any type I ice crystal altitude threshold 120may be used. Further, the type I ice crystal altitude threshold 120 maybe adjustable to account for other conditions. For example, but notlimited to, a first type I ice crystal altitude threshold 120 may beused when the region of airspace 108 potentially having type I icecrystals 110 is over an ocean, and a second type I ice crystal altitudethreshold 120 may be used when the region of airspace 108 potentiallyhaving type I ice crystals 110 is over mountains. The type I ice crystalaltitude threshold 120 may be the same as, or may be different from, theaircraft altitude threshold 118 depending upon the embodiment, and/orbased upon particular weather conditions and/or flight conditions.

Another weather condition that may be considered concurrently with thelow level reflectivity signal strength returns detected by the onboardweather radar system is a convective weather proximity threshold 124.Type I ice crystals may be present when convective weather 122 is nearbysince type I ice crystals are typically generated by liquid waterdroplets that freeze as they are up drafted from the convective weather122. Accordingly, the type I ice crystal display system 100 mayoptionally monitor for convective weather 122 (that has reflectivitysignal strength returns which are greater than the above-described radarreflectivity display threshold).

The convective weather proximity threshold 124 corresponds to adetermined distance between the convective weather 122 and the region ofairspace 108 potentially having type I ice crystals 110. A distance ofthe convective weather 122 from the aircraft 102 may be determined basedon a range (distance from the aircraft 102) of reflectivity signalstrength returns detected by the onboard radar system. Similarly, arange of the region of airspace 108 potentially having type I icecrystals 110 may be determined based on a range of detected reflectivitysignal strength returns. As separation distance between the convectiveweather 122 and the region of airspace 108 potentially having type I icecrystals 110 may be determined based on the differences in determinedranges of the convective weather 122 and the region of airspace 108.Elevations of the aircraft 102, the convective weather 122, and/or theregion of airspace 108 may also be considered in determining theseparation distance between the convective weather 122 and the region ofairspace 108 potentially having type I ice crystals 110.

If the separation distance between the region of airspace 108potentially having type I ice crystals 110 and the convective weather122 is within the convective weather proximity threshold 124, thenembodiments of the type I ice crystal display system 100 may determinethat it is likely that type I ice crystals may be present in theassociated region of airspace 108. For example, but not limited to, ifthe region of airspace 108 from which the low level reflectivity signalstrength returns originated is adjacent to and/or is directly above thedetected convective weather 122, then it is likely that type I icecrystals are present in the associated region of airspace 108. On theother hand, if the region of airspace 108 from which the low levelreflectivity signal strength returns originated is 500 miles away fromthe detected convective weather 122, then it may not be likely that typeI ice crystals are present in the associated region of airspace 108.

However, it is appreciated that prevailing winds may blow a region ofairspace 108 having type I ice crystals 110 a relatively long distancefrom convective weather 122 which generated the type I ice crystals 110.Accordingly, an example embodiment may have the convective weatherproximity threshold set to 100 miles, though any suitable convectiveweather proximity threshold 124 may be used. Further, the convectiveweather proximity threshold 124 may be adjustable to account for otherweather conditions and/or flight conditions. For example, but notlimited to, if prevailing winds are known to be relatively high, theconvective weather proximity threshold 124 may be increased. Similarly,if prevailing winds are known to be relatively low, the convectiveweather proximity threshold 124 may be decreased.

Another weather condition that may be considered concurrently with thelow level reflectivity signal strength returns detected by the onboardweather radar system is a convective weather size threshold. Type I icecrystals 110 may be present when a very large-sized convective weatherformation 122 is present. On the other hand, a relatively small and/orisolated convective weather formation may not be expected to generate asignificant amount of type I ice crystals 110. Accordingly, the type Iice crystal display system 100 may optionally monitor the scope ofreflectivity signal strength returns to determine the size of anydetected convective weather 122. If the size of the convective weather122 is greater than or equal to the convective weather size threshold,then the type I ice crystal display system 100 may determine that it islikely that type I ice crystals 110 may be present in the associatedregion of airspace 108. On the other hand, if the detected convectiveweather 122 is small and/or is isolated, then it is likely that ahazardous level of type I ice crystals 110 is not present in theassociated region of airspace 108. Any suitable convective weather sizethreshold may be used. The convective weather size threshold may bebased on the height, width, estimated vertically integrated liquid,and/or volume of the detected convective weather 122.

Another weather condition that may be considered concurrently with thelow level reflectivity signal strength returns detected by the onboardweather radar system is a convective weather intensity threshold. Moreintense (severe) convective weather 122 is more likely to generate ahazardous concentration of type I ice crystals 110 than less severeconvective weather 122. Accordingly, the type I ice crystal displaysystem 100 may optionally monitor the values of reflectivity signalstrength returns to determine the severity (intensity) of any detectedconvective weather 122. If the intensity of the convective weather 122is greater than or equal to the convective weather intensity threshold,then the type I ice crystal display system 100 may determine that it islikely that type I ice crystals 110 may be present in the associatedregion of airspace 108. Any suitable value for the convective weatherintensity threshold may be used. Further, the value of the convectiveweather intensity threshold may be adjustable to account for otherweather conditions and/or flight conditions. For example, but notlimited to, a first convective weather intensity threshold may be usedfor a particular season and/or a particular geographic area. A secondconvective weather intensity threshold may be used for a differentseason and/or a different geographic area.

Further, the volume or size of portions of the convective weather 122exceeding the convective weather intensity threshold may be considered.For example, similar sized convective weather 122 may exhibit differentsizes or volumes of severe weather. Convective weather 122 having arelatively larger volume or size of severe weather may be more likely togenerate hazardous high concentrations of type I ice crystals 110.Accordingly, embodiments may compare the detected convective weather 122with a minimum volume or size of convective weather having a particularlevel of severity threshold (hereinafter the “volume/severitythreshold”). In the volume or size of a portion of the detectedconvective weather 122 is greater than or equal to the volume/severitythreshold, then the type I ice crystal display system 100 may determinethat it is likely that type I ice crystals 110 may be present in theassociated region of airspace 108.

Another weather condition that may be considered concurrently with thelow level reflectivity signal strength returns detected by the onboardweather radar system is the VIL content value of detected convectiveweather. The VIL content value indicate the amount of water (orprecipitation) in a volume of space, which is typically a column. TheVIL content value may be determined from radar returns that are analyzedin terms of three-dimensional (3-D) space. In some situations, thelocation of the volume of airspace 108 may be spatially removed orseparated from the volume of space for which the VIL content value isdetermined.

If the VIL content value for a particular volume of space is greaterthan or equal to a VIL content value threshold, type I ice crystals maybe present. If the VIL content value is less than or equal to the VILcontent value threshold, then type I ice crystals may not be present inthe region of airspace 108. Accordingly, if a region of airspace 108with low level reflectivity signal strength returns is detected, and atleast one volume of space with a VIL content value is greater than orequal to the VIL content value threshold, then embodiments of the type Iice crystal display system 100 may determine that type I ice crystalsmay be present in the associated region of airspace 108. Any suitableVIL content value threshold may be used.

Further, location of those columns of space with a determinable VILcontent value may be considered when the column of space is within apredetermined range of the aircraft 102. For example, if a location of acolumn of space with a VIL content value (exceeding the VIL contentvalue threshold) is identified at a 5 mile range from the aircraft 102,that particular column may be considered as indicating that the regionof airspace 108 potentially has type I ice crystals 110. On the otherhand, if the column of space with a VIL content value (exceeding the VILcontent value threshold) is identified at a 500 mile range, then thatparticular column may not be considered as indicating that the region ofairspace 108 potentially has type I ice crystals 110. This range fromthe aircraft 102 is referred to herein as a VIL content value rangethreshold. Any suitable VIL content value range threshold may be used bythe various embodiments. Further, the VIL content value range thresholdmay be adjustable by the crew of the aircraft 102, and/or may beautomatically adjustable based on other detected flight conditions. Forexample, the VIL content value range threshold may be adjustable basedon an ambient temperature, terrain conditions, seasonal conditions, orthe like.

Alternatively, or additionally, location of those columns of space witha determinable VIL content value may be considered when the column ofspace is within a predetermined range of the associated region ofairspace 108. For example, if a location of a column of space with a VILcontent value (exceeding the VIL content value threshold) is identifiedat a 1 mile range from the associated region of airspace 108, thatparticular column may be considered as indicating that the region ofairspace 108 potentially has type I ice crystals 110. On the other hand,if the column of space with a VIL content value (exceeding the VILcontent value threshold) is identified at a 10 mile range from theassociated region of airspace 108, then that particular column may notbe considered as indicating that the region of airspace 108 potentiallyhas type I ice crystals 110. This range from the associated region ofairspace 108 may also be referred to herein as a VIL content value rangethreshold. Any suitable VIL content value range threshold may be used bythe various embodiments. Further, the VIL content value range thresholdmay be adjustable by the crew of the aircraft 102, and/or may beautomatically adjustable based on other detected flight conditions. Forexample, the VIL content value range threshold may be adjustable basedon an ambient temperature, terrain conditions, seasonal conditions, orthe like.

FIG. 1 further illustrates a region of airspace 126 having type I icecrystals 110 therein at a range 128 out from the aircraft 102. In somesituations, the region of airspace 126 with type I ice crystals 110 maynot provide a detectable return due to space loss. That is, the radarreturns from the type I ice crystals 110 in the region of airspace 126may become so dispersed by the time that they reach the aircraft 102,that the radar returns may not be detectable by the onboard radarsystem.

In other situations, any detectable returns from the type I ice crystals110 in the region of airspace 126 may be below a minimum radarreflectivity display threshold. The minimum radar reflectivity displaythreshold may be set to reduce nuisance alarms or to not display suchregion of airspace 126 that are so far removed from the aircraft 102 asto not constitute a significant hazard to the aircraft 102. Embodimentsmay optionally employ the minimum radar reflectivity display threshold.Any suitable value for the minimum radar reflectivity display thresholdmay be used, such as 5 dBZ.

Further, the range 128 of the region of airspace 126 is greater than themaximum range threshold 114. Accordingly, even if the region of airspace126 having the type I ice crystals 110 is detected, embodiments of thetype I ice crystal display system 100 may be configured to not alert thecrew or otherwise display the region of airspace 126 on the display ofthe onboard weather radar system. The region of airspace 126 may not bedisplayed because it is too far away to be a credible hazard to theaircraft 102.

Summarizing, embodiments 100 the type I ice crystal display system 100are configured to conditionally detect regions of airspace 108potentially having type I ice crystals 110 based on a detected lowreflectivity signal strength return values that are less than or equalto a radar reflectivity display threshold. If other weather and/orflight conditions exist concurrently with detection of a region ofairspace 108 potentially having type I ice crystals 110, then the crewof the aircraft are alerted or warned, and/or the region of airspace 108is indicated on a display of the onboard weather radar system. Examplesof conditional testing wherein embodiments of the type I ice crystaldisplay system 100 will identify regions of airspace 108 potentiallyhaving type I ice crystals 110, and then notify the crew with an audiblealert or display, include but are not limited to:

1. Low Reflectivity signal strength returns conditionally tested withAmbient Temperature less than or equal to an ambient air temperaturethreshold.

2. Low Reflectivity signal strength returns conditionally tested withaircraft altitude greater than or equal to an aircraft altitudethreshold 118.

3. Low Reflectivity signal strength returns conditionally tested with analtitude of the region of airspace 108 greater than or equal to a type Iice crystal altitude threshold 120.

4. Low Reflectivity signal strength returns conditionally tested withambient temperature being less than or equal to an ambient airtemperature threshold, and conditionally tested with range of theairspace 108 from the aircraft 102 being less than or equal to a maximumrange threshold 114.

5. Low Reflectivity signal strength returns conditionally tested withthe altitude of the aircraft 102 being greater than or equal to theaircraft altitude threshold 118, and conditionally tested with a rangeof the airspace 108 from the aircraft 102 being less than or equal to amaximum range threshold 114.

6. Low Reflectivity signal strength returns conditionally tested withthe altitude of the region of airspace 108 being greater than or equalto a type I ice crystal altitude threshold 120, and conditionally testedwith a range of the airspace 108 from the aircraft 102 being less thanor equal to a maximum range threshold 114.

7. Low Reflectivity signal strength returns conditionally tested withambient temperature being less than or equal to an ambient airtemperature threshold, and conditionally tested with range of theairspace 108 from the aircraft 102 being greater than or equal to aminimum range threshold 116.

8. Low Reflectivity signal strength returns conditionally tested withthe altitude of the aircraft 102 being greater than or equal to theaircraft altitude threshold 118, and conditionally tested with range ofthe airspace 108 from the aircraft 102 being greater than or equal to aminimum range threshold 116.

9. Low Reflectivity signal strength returns conditionally tested withaltitude of the region of airspace 108 being greater than or equal to atype I ice crystal altitude threshold 120, and conditionally tested withrange of the airspace 108 from the aircraft 102 being greater than orequal to a minimum range threshold 116.

10. Low Reflectivity signal strength returns conditionally tested withambient temperature being less than or equal to an ambient airtemperature threshold, and conditionally tested with a distance betweenthe region of airspace 108 and the detected convective weather 122 beingless than or equal to a convective weather proximity threshold 124.

11. Low Reflectivity signal strength returns conditionally tested withambient temperature being less than or equal to an ambient airtemperature threshold, and conditionally tested with at least one sizecharacteristic of the convective weather 122 being greater than or equalto a convective weather size threshold.

12. Low Reflectivity signal strength returns conditionally tested withat least one proximate column of airspace having a VIL content beinggreater than or equal to a VIL content threshold, and optionallyconditionally tested with a location of the column of airspace beinglocation being at a range that is less than or equal to a VIL contentrange threshold.

The above described conditional testing performed by various embodimentsof the type I ice crystal display system 100 are non-limiting examples.Embodiments may use any combination of, and any number of, conditionalthresholds so as to reliably identify regions of airspace 108potentially having type I ice crystals 110.

Other combinations of weather conditions and/or flight conditions may beused by alternative embodiments of the type I ice crystal display system100 to conditionally detect region of airspace 108 potentially havingtype I ice crystals 110 based on a detected low reflectivity signalstrength return values. All such variations are intended to be withinthe scope of this disclosure and protected by the accompanying claims.

FIG. 2 is a block diagram of an embodiment of the type I ice crystaldisplay system 100 implemented in an aviation electronics system 202 ofthe aircraft 102. The aviation electronics system 202 includes a globalpositioning system (GPS) 204, a transceiver 206, an inertial measurementunit (IMU) 208, an onboard weather radar system 210, a processing system212, a display system 214, a memory 216, and a crew interface 218. Theonboard weather radar system 210 includes an antenna 220 that isoperable to emit radar signals and receive radar returns. The displaysystem 214 includes a display 222. It is appreciated that the aviationelectronics system 202 includes many other components and/or systemsthat are not illustrated or described herein.

The above-described components, in an exemplary embodiment, arecommunicatively coupled together via communication bus 224. Inalternative embodiments of the aviation electronics system 202, theabove-described components may be communicatively coupled to each otherin a different manner. For example, one or more of the above-describedcomponents may be directly coupled to each other, or may be coupled toeach other via intermediary components (not shown).

The radar system 210 may be any suitable onboard weather radar system,such as, but not limited to, a weather radar that is operable to detectweather that is located relatively far away from the aircraft 102. Theantenna 220 is operable to emit radar pulses and to sense the signalstrength of received radar returns. A radar return is reflected energyfrom an object, such as ice crystals and/or water droplets, upon whichthe emitted radar pulse is incident on. The antenna 220 is swept in aback-and-forth motion, in an up and down direction, and/or in otherdirections of interest, such that the radar system 210 is able to detectweather, and more particularly type I ice crystals 110, in an area ofinterest about the aircraft 102. Embodiments of the type I ice crystaldisplay system 100 may be implemented in other types and/or applicationsof radar.

An exemplary embodiment of the type I ice crystal display system 100comprises a plurality of cooperatively acting modules. The modules areidentified as a radar information processing module 226, a flight planprocessing module 228, an optional vertical display processing module230, a type I ice crystals detecting module 232, and a type I icecrystals airspace region notification module 234. Modules 226, 228, 230,232, 234 reside in the memory 216, and are retrieved and executed by theprocessing system 212. In an exemplary embodiment, a conditions database236 is stored in memory 216. In other embodiments, the modules 226, 228,230, 232, 234 may be implemented together as a common module, may beintegrated into other modules, or reside in other memories (not shown).Further, the conditions database 236 may be implemented with otherdatabases, may be implemented in various formats, such as a buffer orthe like, and/or may be implemented in another memory.

The radar information processing module 226 processes informationcorresponding to the received radar returns detected by the antenna 220of the onboard weather radar system 210. Various types of weather, andtheir associated attributes, are determined by the radar informationprocessing module 226. More particularly, weather information isdetermined for the detected radar return levels. Selected determinedweather information may be optionally saved into the correspondingvoxels of 3-D weather information database (not shown).

Based on received radar returns, the radar information processing module226 may optionally determine VIL content for columns of airspace in theregion of airspace 104. VIL content may be determined for columns ofairspace having any suitable volume, footprint area, and/or height. Anysuitable method, process or apparatus may be employed by the variousembodiments to determine VIL content.

The flight plan processing module 228 processes flight plan information.Flight plans may be predefined and/or entered by the crew. A predefinedflight plan typically comprises a plurality of planned flight pathsegments based upon a series of waypoints. Planned flight path segmentsmay be straight or curvilinear. An example flight path 112 isillustrated in FIG. 1. The flight plan information includes geographiclocation information that defines location of waypoints and/or theflight path segments, and planned altitude information. The flight planinformation may optionally include various limits, such as altitudefloors, altitude ceilings, and/or exclusion regions or zones. In someembodiments, the flight plan may be dynamically adjusted during flightbased upon crew input, based upon current location of the aircraft 102as provided by the GPS 204 and/or the IMU 208, and/or based uponinstructions or information received by the transceiver 206.

The vertical display processing module 230 retrieves weatherinformation, and more particularly the location of any identifiedregions of airspace 108 potentially having type I ice crystals 110,along a predefined or selected vertical plane, such as the verticalslice 130 (FIG. 1). The vertical slice weather information iscommunicated to the display system 214, which displays an imagecorresponding to the vertical slice 130 showing the location and thealtitude of any identified regions of airspace 108 potentially havingtype I ice crystals 110.

The type I ice crystals detecting module 232 accesses the weatherinformation provided by the onboard weather radar system 210, namely theradar return intensity information and the associated locationinformation, and identifies region of airspace 108 potentially havingtype I ice crystals 110 have been detected. Thus, the type I icecrystals detecting module 232 identifies regions of airspace exhibitinglow values of radar return intensities, and based upon other weatherconditions and/or aircraft conditions, determines if the identifiedairspace is likely to contain type I ice crystals 110.

Information pertaining to the various weather condition and/or aircraftcondition thresholds is stored in the conditions database 236.Information about the various conditions may be predefined or may beselectably specified by the crew of the aircraft 102 via input providedvia the crew interface 218. For example, but not limited to, the crewmay adjust the maximum range threshold 114 and/or the minimum rangethreshold 116 by providing suitable inputs to the crew interface 218.

Once a region of airspace 108 potentially having type I ice crystals 110is identified, then information pertaining to the identified region ofairspace 108 potentially having type I ice crystals 110 is communicatedto the type I ice crystals airspace region notification module 234. Thetype I ice crystals airspace region notification module 234 determineshow the information is presented to the crew of the aircraft 102.

In some situations, an audible notification, warning, alarm or the likeis issued to the crew. Accordingly, the type I ice crystals airspaceregion notification module 234 generates information that causescommunication of sound-based information to an audio system (not shown)that emits the audible notification, warning, alarm or the like.

Additionally, or alternatively, the presented images on the display 222of the display system 214 are modified to indicate the presence of theregion of airspace 108 potentially having type I ice crystals 110.Accordingly, the type I ice crystals airspace region notification module234 communicates graphics-based information that is used to modifyimages of detected weather that is presented on the display 222.

FIG. 3 is a display image 300 presenting a plan view of the plannedflight path 112 through the region of airspace 104 having convectiveweather 122 prior to detection of the region of airspace 108 potentiallyhaving type I ice crystals 110. FIG. 4 is a display image 400 presentinga plan view of the planned flight path 112 through the region ofairspace 104 having convective weather 122 after to detection of theregion of airspace 108 potentially having type I ice crystals 110.

The plan views 300 and 400 display the graphical information, such asicons or the like, indicating the location of the convective weather122. Similar to FIG. 1, reference numerals of the icons of FIGS. 3 and 4correspond to the reference numerals of the convective weather 122and/or the region of airspace 108 potentially having type I ice crystals110 of FIG. 1 for convenience.

In the exemplary plan views 300 and 400, the extent of the convectiveweather 122 is indicated by the bold outlined graphical region 302.Various levels of severity of the convective weather 122 may beindicated using graphical information presented on the display 222. Inthe simplified plan views 300 and 400, less severe weather is indicatedusing a white background, such as noted in the periphery area bounded bythe outlined graphical region 302. Moderately severe weather isindicated using a shaded background, such as noted in the shadedperiphery area bounded by the outlined graphical region 304. Very severeweather is indicated using a black background, such as noted in theblack periphery area bounded by the outlined graphical region 306. Thus,the crew of the aircraft 102 appreciate that the planned flight path 112will traverse through the convective weather 122. Further, the planviews 300 and 400 indicate that if the very severe weather region is atthe same altitude of the aircraft 102, that the aircraft will traversethrough this very severe weather region of the convective weather 122.(Some onboard weather radar systems 210 are configured to present avertical slice view which indicates the altitude of various portions ofthe convective weather 122 along a selected vertical slice 130illustrated in FIG. 1.)

The simplified plan views 300 and 400 indicate the graphical regions302, 304, 306 using different shading. In practice these graphicalregions 302, 304, 306 corresponding to differing levels of weatherseverity would be presented on the display 222 using different colors.For example, the less severe weather regions may be indicated using awhite color. Moderately severe weather regions may be indicated using ayellow color, wherein the yellow color indicates an alert condition tothe crew. Very severe weather regions may be indicated using a redcolor, wherein the red color indicates a warning condition to the crew.Accordingly, the crew may choose to modify their planned flight path 112so as to avoid the airspace regions indicating severe and/or moderateweather conditions. Other onboard weather radar systems 210 may usedifferent graphics to indicate weather, such as by using differentcolors or the like to indicate differing levels of weather severity.

In the simplified plan view 300, the region of airspace 108 potentiallyhaving type I ice crystals 110 is not indicated. However, as some point,the onboard weather radar system 210 will detect low level radar returnsthat may be associated with type I ice crystals 110. If at least oneother weather condition and/or flight condition is also met, then thetype I ice crystal display system 100 may identify the region ofairspace 108 as potentially having type I ice crystals 110. Upon theidentification of the region of airspace 108 potentially having type Iice crystals 110, embodiments then take actions to advise the crew ofthe aircraft 102 of the detection of the region of airspace 108potentially having type I ice crystals 110. As noted above, embodimentsmay issue an audible advisory or warning to the crew of the aircraft102.

Alternatively, or additionally, the detected region of airspace 108potentially having type I ice crystals 110 may be indicated on thedisplay 222. To illustrate, the simplified plan view 400 shows agraphical region 402 which corresponds to the detected region ofairspace 108 potentially having type I ice crystals 110. The graphicalregion 402 is illustrated as a cross-hatched area so as to differentiatefrom the regions corresponding to the detected convective weather 122.Accordingly, after the presented images on the display 22 changed fromthe plan view 300 of FIG. 3 to the plan view 400 of FIG. 4, the crew ofthe aircraft 102 will appreciate the existence of the region of airspace108 potentially having type I ice crystals 110. The crew may then chooseto fly through the region of airspace 108 potentially having type I icecrystals 110, or the crew may choose to modify their planned flight pathso as to avoid the region of airspace 108 potentially having type I icecrystals 110.

In an example embodiment, the graphical region 402 corresponding to theregion of airspace 108 potentially having type I ice crystals 110 wouldbe presented on the display 222 using a blue, or substantially blue,color. Accordingly, the crew will readily appreciate the nature of theweather (the region of airspace 108 potentially having type I icecrystals 110) associated with the presented graphical region 402, andwill readily appreciate the differences from the presented graphicalregions 302, 304, 306.

Alternatively, or additionally, the color and/or intensity of thepresented graphical region 402 may varied to indicate the severity,and/or the level of risk, associated with the type I ice crystals 110.For example, but not limited to, if the concentration of type I icecrystals 110 in the region of airspace 108 is relatively high, thegraphical region 402 may be presented using a very brilliant color, suchas, but not limited to, a brilliant blue color. Conversely, if theconcentration of type I ice crystals 110 in the region of airspace 108is relatively low, the graphical region 402 may be presented using arelatively pale color, such as, but not limited to, a pale blue color.The color intensity may be slidably adjusted so as to be able toindicate different levels of severity. In some embodiments, differentcolors may be used to indicate different levels of severity of the ifthe amount of type I ice crystals 110 in the region of airspace 108.

Concentration of type I ice crystals 110 may be determined based on thesignal strength of the received radar signal returns. A higher radarreflectivity signal strength return corresponds to a higherconcentration of type I ice crystals 110. Further, the range of theregion of airspace 108 may be additionally considered. For example,reflectivity signal strength returns of a particular level originatingfrom a closer region of airspace 108 corresponds to a higherconcentration of type I ice crystals 110. In some embodiment, the weightof significance of radar reflectivity signal strength returns and/orranges may be slidably adjusted so as to be able to indicate differentconcentrations of type I ice crystals 110. In some embodiments,different concentrations may be used to indicate different levels ofseverity of the if the amount of type I ice crystals 110 in the regionof airspace 108

Alternatively, or additionally, the graphical region 402 may beindicated in a flashing manner. That is, the graphical region 402 wouldbe repetitively presented for a brief first duration, and then notdisplayed for a second duration. For example, the graphical region 402may be presented every other second on the display 222. Further,frequency of the flashing of the graphical region 402 may indicate theseverity, and/or the level of risk, associated with the type I icecrystals 110. For example, but not limited to, if the concentration oftype I ice crystals 110 in the region of airspace 108 is relativelyhigh, the graphical region 402 may be flashed at a faster rate.Conversely, if the concentration of type I ice crystals 110 in theregion of airspace 108 is relatively low, the graphical region 402 maybe flashed at a slower rate. The flashing rate may be slidably adjustedso as to be able to indicate different levels of severity.

While the preferred embodiment of the invention has been illustrated anddescribed, as noted above, many changes can be made without departingfrom the spirit and scope of the invention. Accordingly, the scope ofthe invention is not limited by the disclosure of the preferredembodiment. Instead, the invention should be determined entirely byreference to the claims that follow.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A method of detecting type I ice crystals using an onboard weather radar system of an aircraft, the method comprising: identifying radar returns having a return level signal strength less than a radar return sensitivity threshold level; determining if at least one of a weather condition and a flight condition concurrently exists with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level; and identifying a region of airspace potentially having type I ice crystals when the at least one of the weather condition and the flight condition concurrently exists with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level.
 2. The method of claim 1, wherein the type I ice crystals are found when the ambient temperature is between −5° C. to −55° C.
 3. The method of claim 1, further comprising: displaying on a display information identifying the region of airspace potentially having type I ice crystals in response to identifying the region of airspace potentially having type I ice crystals.
 4. The method of claim 1, wherein the at least one of the weather condition and the flight condition is an altitude of the aircraft, and wherein the determining further comprises: comparing the altitude of the aircraft with an altitude threshold, wherein the at least one of the weather condition and the flight condition is determined to concurrently exist with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level when the altitude of the aircraft is at least equal to the altitude threshold.
 5. The method of claim 4, further comprising: determining a range of the aircraft from the identified region of airspace potentially having type I ice crystals; and comparing the range of the aircraft from the identified region of airspace potentially having type I ice crystals with a maximum range threshold, wherein the at least one of the weather condition and the flight condition is determined to concurrently exist with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level when the range of the aircraft from the identified region of airspace potentially having type I ice crystals is less than the maximum range threshold.
 6. The method of claim 4, further comprising: determining a range of the aircraft from the identified region of airspace potentially having type I ice crystals; and comparing the range of the aircraft from the identified region of airspace potentially having type I ice crystals with a minimum range threshold, wherein the at least one of the weather condition and the flight condition is determined to concurrently exist with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level when the range of the aircraft from the identified region of airspace potentially having type I ice crystals is greater than the minimum range threshold.
 7. The method of claim 1, wherein the at least one of the weather condition and the flight condition is an altitude of the identified region of airspace potentially having type I ice crystals, and wherein the determining further comprises: comparing the altitude of the identified region of airspace potentially having type I ice crystals with a type I ice crystal altitude threshold, wherein the at least one of the weather condition and the flight condition is determined to concurrently exist with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level when the altitude of the identified region of airspace potentially having type I ice crystals is at least equal to the type I ice crystal altitude threshold.
 8. The method of claim 1, wherein the at least one of the weather condition and the flight condition is a vertically integrated liquid-water (VIL) content value of a volume of airspace, and wherein the determining further comprises: comparing the VIL content value of the volume of airspace with a VIL content value threshold, wherein the at least one of the weather condition and the flight condition is determined to concurrently exist with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level when the altitude of the identified region of airspace potentially having type I ice crystals is at least equal to the type I ice crystal altitude threshold.
 9. The method of claim 8, further comprising: comparing a range of the volume of airspace with the VIL content value at least equal to the VIL content value threshold with a VIL content range threshold, wherein the at least one of the weather condition and the flight condition is determined to concurrently exist with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level when the range of the volume of airspace with the VIL content value at least equal to the VIL content value threshold is less than the VIL content range threshold.
 10. The method of claim 1, wherein the at least one of the weather condition and the flight condition is an estimated temperature for at least one of a location of and an altitude of the region of airspace potentially having type I ice crystals, and wherein the determining further comprises: comparing the estimated temperature outside of the aircraft with an ambient temperature threshold, wherein the at least one of the weather condition and the flight condition is determined to concurrently exist with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level when the ambient temperature outside of the aircraft is at least equal to the ambient temperature threshold.
 11. The method of claim 1, wherein the at least one of the weather condition and the flight condition is an ambient temperature outside of the aircraft, and wherein the determining further comprises: comparing the ambient temperature outside of the aircraft with an ambient temperature threshold, wherein the at least one of the weather condition and the flight condition is determined to concurrently exist with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level when the ambient temperature outside of the aircraft is at least equal to the ambient temperature threshold.
 12. The method of claim 11, further comprising: determining a range of the aircraft from a location of the identified region of airspace potentially having type I ice crystals; and comparing the range of the aircraft from the identified region of airspace potentially having type I ice crystals with a maximum range threshold, wherein the at least one of the weather condition and the flight condition is determined to concurrently exist with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level when the range of the aircraft from the identified region of airspace potentially having type I ice crystals is less than the maximum range threshold.
 13. The method of claim 11, further comprising: determining a range of the aircraft from a location of the identified region of airspace potentially having type I ice crystals; and comparing the range of the aircraft from the identified region of airspace potentially having type I ice crystals with a minimum range threshold, wherein the at least one of the weather condition and the flight condition is determined to concurrently exist with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level when the range of the aircraft from the identified region of airspace potentially having type I ice crystals is greater than the minimum range threshold.
 14. The method of claim 1, wherein the at least one of the weather condition and the flight condition is a size of convective weather detected by an onboard radar system of the aircraft, and wherein the determining further comprises: comparing the size of the convective weather with a convective weather size threshold, wherein the at least one of the weather condition and the flight condition is determined to concurrently exist with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level when the size of the convective weather is at least equal to the convective weather size threshold.
 15. The method of claim 14, further comprising: determining a range of the aircraft from a location of the convective weather; and comparing the range of the aircraft from the convective weather with a convective weather proximity threshold, wherein the at least one of the weather condition and the flight condition is determined to concurrently exist with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level when the range of the aircraft from the convective weather is less than the convective weather proximity threshold.
 16. A system onboard an aircraft, comprising: an onboard radar system configured to receive radar returns from weather in proximity to the aircraft; a display system configured to display information corresponding to the received radar returns; and a processing system communicatively coupled to the onboard radar system and the display system, and configured to: identify radar returns having a return level signal strength less than a radar return sensitivity threshold level; determine if at least one of a weather condition and a flight condition concurrently exists with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level; and identify a region of airspace potentially having type I ice crystals when the at least one of the weather condition and the flight condition concurrently exists with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level, wherein the region of airspace potentially having type I ice crystals is displayed on the display.
 17. The system of claim 16, wherein the at least one of the weather condition and the flight condition is an altitude of the aircraft, and wherein the processing system is further configured to: compare the altitude of the aircraft with an altitude threshold, wherein the at least one of the weather condition and the flight condition is determined to concurrently exist with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level when the altitude of the aircraft is at least equal to the altitude threshold.
 18. The system of claim 16, wherein the at least one of the weather condition and the flight condition is an altitude of the identified region of airspace potentially having type I ice crystals, and wherein the processing system is further configured to: compare the altitude of the identified region of airspace potentially having type I ice crystals with a type I ice crystal altitude threshold, wherein the at least one of the weather condition and the flight condition is determined to concurrently exist with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level when the altitude of the identified region of airspace potentially having type I ice crystals is at least equal to the type I ice crystal altitude threshold.
 19. The system of claim 16, wherein the at least one of the weather condition and the flight condition is an ambient temperature outside of the aircraft, and wherein the processing system is further configured to: compare the ambient temperature outside of the aircraft with an ambient temperature threshold, wherein the at least one of the weather condition and the flight condition is determined to concurrently exist with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level when the ambient temperature outside of the aircraft is at least equal to the ambient temperature threshold.
 20. A method of detecting type I ice crystals using an onboard weather radar system of an aircraft, the method comprising: receiving radar returns at the onboard weather radar system; identifying radar returns having a return level signal strength less than a radar return sensitivity threshold level; comparing at least one of a weather condition and a flight condition with a corresponding threshold, wherein the at least one of the weather condition and the flight condition is at least one of: an altitude of the aircraft, wherein the threshold is an altitude threshold; an altitude of the identified region of airspace potentially having type I ice crystals, wherein the threshold is a type I ice crystal altitude threshold; a first range of the aircraft from a location of convective weather, wherein the threshold is a first range threshold; a second range of the aircraft from a location of the radar returns having the return level signal strength less than the radar return sensitivity threshold level, wherein the threshold is a second range threshold; an ambient temperature outside of the aircraft, wherein the threshold is an ambient temperature threshold; and a vertically integrated liquid-water (VIL) content value of a volume of airspace associated with the radar returns having the return level signal strength less than the radar return sensitivity threshold level wherein the threshold is a VIL content value threshold; determining if the at least one of the weather condition and the flight condition concurrently exists with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level; and identifying a region of airspace as having type I ice crystals when the at least one of the weather condition and the flight condition concurrently exists with the identified radar returns having the return level signal strength less than the radar return sensitivity threshold level, wherein the type I ice crystals are found when the ambient temperature is between −5° C. to −55° C. 